You can sow seeds in as little as a week or even sooner after spraying glyphosate, a systemic, nonselective weed killer. Glyphosate moves from the leaves to the roots of plants, destroying the entire plant, but leaving no residue in the soil. The chemical affects many types of plants, including weeds, grasses and desirable plants, but after the liquid is absorbed into the plant, it doesn’t pose any further threat. You can safely sow ornamental flower seeds a day after spraying with glyphosate and grass and vegetable seeds, three days after, even though the herbicide takes up to seven days to destroy weeds. If you remove the dying weeds too soon, live roots could remain in the soil, ready to regrow. Another systemic weed killer that doesn’t affect seeds is pelargonic acid.
It makes sense to be cautious about sowing seed after using weed killer. Certain herbicides can harm sprouting seeds and young plants. However, while you must wait several months to sow seed after applying some weed killers, you only need to wait a few days after applying others. The reason for this difference lies in the effect of the active chemicals in the individual products. Read the label carefully and follow all the directions when applying a weed killer.
Pre-Emergence Weed Killers and Sowing Seed
Sowing seed after applying a pre-emergence weed killer disturbs the chemical barrier on the soil surface, which means that weed seeds may germinate too.
Pre-emergence weed killers prevent seeds from sprouting. They create a chemical barrier on the soil surface that suppresses seed development. What this means is, if you sow your own seed after applying a pre-emergence weed killer, the seed isn’t likely to grow. However, some pre-emergence products only affect grassy weeds, so you can safely sow most vegetable and flower seeds after applying these herbicides. The same doesn’t apply to reseeding or overseeding your lawn. Grass seed won’t sprout until a pre-emergence weed killer has decayed and become ineffective. For example, it isn’t safe to sow lawn seed until four months after applying a crabgrass preventer.
Many selective weed killers leave little or no trace in the soil, and they target certain plants while leaving others unharmed. Generally, these types of herbicides destroy either grassy weeds or broadleaf weeds. You can safely sow most seeds in your vegetable or flower patch a day after applying selective herbicides, such as sethoxydim, clethodim and bentazon, for grassy weeds. These herbicides only affect your desired plants if the plants belong to the grass family. For lawns, herbicides that destroy broadleaf weeds are effective, but it isn’t safe to reseed until a month after applying these products, unless the label states differently.
Water deeply (remember the time it took for whatever irrigation method you are using). Use a shovel, dig down to see the soil profile. Simply jab the shovel into the lawn and pull back exposing 4″ of lawn bed, the soil. When you see water reaching down 4″ beneath the surface that is the correct amount of time.
The next critical thing you need to do once per year is aerating! Pulling plugs of sod and soil out of the lawn then allowing them to stay where they fall. This is another piece of equipment to easily rent. Share this expense with your neighbors and have an annual PARTY!
If you simply kill the area with a weedkiller, wait 4-6 weeks (which is really the minimum time) and re seed, you’re likely to have a lot of weed regrowth – no matter what the manufacturers may claim, weedkillers aren’t always effective at first go, and don’t work perfectly on everything anyway. Again, despite the claims, Round up does leave a residue in soil which may be taken up by growing plants, although less likely to affect grass seed spread after 4-6 weeks. Your comment that you think the soil beneath the grass is ‘rocky and weak’, if an accurate observation, means that, unless you correct the soil beneath, you’ll end up with a similar lawn to the one you’ve got now, despite adding topsoil and seed. Topsoil brings its own disadvantages – unless you pay top dollar, its often full of weed seeds itself, so its worth checking that you’re getting good stuff, not just any old unscreened motorway spoil.
5 Answers 5
Just to throw my 2 cents in on this old thread that someone may find like I did, don’t forget to water the weed seeds. This is a hard lesson I learned converting an old field into wildflowers. An applications of glyphosate would kill the plants the spray touched, but those plants were sheltering seeds and seedlings under them, which took off once the competition was gone. We sprayed the new seedlings and killed them off. When it was all brown with no green we burned it all with propane, creating a beautiful black bed for our wildflower seeds. Big mistake. As soon as we started to water the wildflower seeds, the dormant weed seeds in the ground took off.
If the “lawn” is more weed than grass, I would kill everything and start over. If you take that option, Ortho is the wrong product because it isn’t supposed to kill grass.
The best next step for you is to rent a ‘sod cutter’ from an equipment rental store. Easy peasy to use, cheap and it will cut out all of the soil and weed crowns like now. 2″ deep. or 1 1/2 inches. These pieces of sod make valuable soil for plant beds. Just pile them up upside down then cover with 4 inches of soil. Don’t send this stuff to the dump.
I’d be more inclined to use sod rather than seed. Far better to discourage any weed seed germinating during this time.